Most, if not all, nuclear power plants use the heat which is generated in the nuclear reactor to generate steam which is used to provide mechanical energy. The heating medium such as liquid sodium or hot gas which is heated by the heat energy generated in the nuclear reactor is flowed through a steam generator in which water to be heated and/or steam to be reheated is flowed through tubes while the heating medium flows over the tubes so that the heating medium heats the fluid flowing within the tubes. Often different tubes are provided for heating the feedwater (economizer tubes), generating steam from the water (evaporator tubes) and for superheating the steam which is generated in the evaporator tubes, (superheater tubes). The tubes may be helical coil tubes, or straight tubes or bent tubes, or they may be bayonet tube assemblies. Usually the tubes are connected with one or more tube sheets so that the heating medium and the fluid which flows through the tubes are separated.
An example of a steam generator which uses bayonet tubes and all of which are connected with a single tube sheet is found in co-pending U.S. Pat. application Ser. No. 486,286, which was filed on July 8, 1974. There, tubes used for different phases of steam generation such as economizer tubes, evaporator tubes and superheater tubes are connected to the same tube sheet. Chambers generally below that tube sheet, allow fluid leaving the economizer bayonet tube assemblies to flow into the evaporator tube assemblies and to flow from the evaporator tube assemblies to the superheater tube assemblies. While this arrangement is satisfactory from a standpoint of efficiency, thermal balance and energy output there are certain situations in which construction of such a steam generator would be rather expensive. For example, in a large steam generator the chambers below the tube sheet will hold steam at high pressure and consequently the tube sheet and the associated structure necessary to define the chambers will have to be of sufficient dimensions to withstand the high pressures over a large area. A tube sheet is strengthened by making it thicker. This not only adds to the cost and weight of the steam generator, but also makes for a steam generator which will not withstand large temperature gradients over the tube sheet without cracking.
Further, tube sheet failure results in leakage of steam up through the tube sheet to mix with the heating medium and after the pressure above and below tube sheet equalize the heating medium will contaminate the steam. A subsequent failure below the tube sheet would then result in the heating medium being released. This is not acceptable from a safety viewpoint.